The present invention relates to an amplification circuit, and more specifically, to an analog multistage amplification circuit for amplifying a small input signal.
Recently, sensors are used in various fields, and sensors are being miniaturized. Miniaturized sensors generate small output signals. Therefore, an analog multistage amplification circuit, which amplifies the small output signal of a sensor, must have an improved S/N ratio.
FIG. 1 is a schematic circuit diagram showing an analog multistage amplification circuit 100 for amplifying a small input signal Vin output from a device such as a sensor. Input stage and output stage amplifiers 1 and 2, which are connected in series, amplify the input signal Vin and generate an output signal Vout. A high-pass filter 3 is connected between the input stage and the output stage amplifiers 1 and 2 to remove low frequency noise from an output signal of the input stage amplifier 1.
The total gain of the amplification circuit 100, which corresponds to a value obtained by multiplying the gain of the amplifier 1 and the gain of the amplifier 2, is set to be constant. In order to improve the S/N ratio of the amplification circuit 100, the gain of the amplifier 1 is set to a relatively large value and the high-pass filter 3 attenuates noise components contained in the output signal of the amplifier 1. It is desirable that the filtered output signal then be amplified so that the total gain becomes constant in the amplifier 2.
However, if the gain of the amplifier 1 is increased, a saturated state, in which the amplitude of the output signal of the amplifier 1 exceeds the operation voltage range, may occur when the amplitude of the input signal increases. In such a case, stable amplification cannot be performed. Therefore, the gain of the amplifier 1 must be set to a value applicable to the maximum value of the tolerable input signal. Further, the gain of the amplifier 2 must be set to a value ensuring the total gain. When the gain is set in such a manner, the gain of the amplifier 1 cannot be set to a sufficiently large value, and the gain of the amplifier 2 cannot be set to a small value. Consequently, noise components provided to the amplifier 2 cannot be sufficiently attenuated, and the S/N ratio of the output signal Vout cannot be increased.
Japanese Laid-Open Patent Publication no. 6-138885 describes a gain control unit for keeping the total gain constant (refer to FIG. 2 of the publication). That is, the gain control unit controls the gain of a variable gain amplifier and an input value of a scaling unit based on an output signal of an AD converter. When the gain of the variable gain amplifier is set to n times, the input value of the scaling unit is set to 1/n times. Through such control, the gain of the variable gain amplifier is adjusted so that it is included in the dynamic range of the AD converter, and the gain is controlled to be constant from an error microphone to an adaptive signal processor.
Japanese Laid-Open Patent Publication No. 11-154839 describes a configuration for setting a constant total gain for a plurality of variable gain amplifiers.
Japanese Laid-Open Patent Publication No. 2004-343539 describes an example of a gain adjustable differential input amplifier.